Artificial reverberation unit



Oct. 8, 1963 A. c. YOUNG ARTIFICIAL REVERBERATION UNIT 2 Sheets-Sheet 1 FiledJan. so, 1961 Oct. 8, 1963 A. c. YOUNG ARTIFICIAL REVER BERATION UNIT FiledJan. 50, 1961 2 Sheets-Sheet 2 4o 6658 oJ United States Patent 3,106,610 ARTIFICIAL REVEREERATION UNIT Alan C. Young, Des I'laines, lll., assignor to Hammond Organ Company, Chicago, Ill., a corporation. of Dela- Ware Filed Jan. 30, 1%1, Ser. No. 85,515 1 Claim. (Cl. 1791) The present invention relates to artificial reverberation apparatus and may, for some purposes at least, be considered as an improvement over the somewhat similar apparatus which forms the subject matter of United States patent application Serial No. 766,840, filed October 13, 1958, now Patent number 2,982,819, for Artificial Reverberation Apparatus, by Herbert E. Meinema and Herbert H. Canfield.

Artificial reverberation apparatus of the general type to which this invention is directed is for the purpose of supplying reverberation for a sound signal from a record player, radio, musical instrument of the electrical type, or the like, the ultimate objective being to simulate the reverberative effect of a room or hall of appropriate acoustical properties within a relatively small listening space.

One of the objectives of the present invention is to provide novel reverberation apparatus of this character at considerably lower cost than has heretofore been considered feasible, thereby extending the utility of the apparatus.

An additional objective is to provide novel artificial reverberation apparatus of high quality which has fewer installation limitations, since the apparatus is smaller, relatively insensitive to attitude, and less sensitive to outside vibration.

Yet another object is to provide novel artificial reverberation apparatus of the type generally outlined above which has a higher order of efficiency and which introduces very little distortion into the system.

Still another object is to provide improved artificial reverberation apparatus which is smaller and which has fewer elements than have heretofore been considered to be necessary.

Other objects and advantages will become apparent from the following description of a preferred embodiment of my invention which is illustrated in the accompanying drawings.

In the drawings, in which similar characters of reference refer to similar parts throughout the several views,

FIG. 1 is a front view of the reverberation apparatus shown in one attitude it might assume when installed;

FIG. 2 is an enlarged plan view of a pair of reverberative spring elements forming a portion of the apparatus of FIG. 1;

FIG. 3 is an enlarged view of the pickup transducer portion of the apparatus shown partially in plan and partially in longitudinal medial section;

FIG. 4 is a transverse sectional view which may be considered as taken in the direction of the arrows substantially along the line 4-4 of FIG. '1;

FIG. 5 is a transverse sectional view which may be considered as taken in the direction of the arrows substantially along the line 5-5 of FIG. 1; and

FIG. 6 is a greatly enlarged elevational view of a detail of the apparatus.

The apparatus of the previously referred to patent application admirably serves its purpose, but is subject to some disadvantages which it is the principal objective of this invention to overcome. That apparatus, although much more compact than any previously known so far as I am aware, is nevertheless large enough to occasion some installation problems, and furthermore it is sensi- 3,1 96,6 l ll Patented Oct. 8, 1 963 tive to attitude, since the reverberating springs are draped in catenary curves between the driver and pickup elements. It is also sensitive to slight rocking movement of the cabinet in which it is installed, since the springs are free to sway and may engage each other or other portions of the structure, thereby introducing serious distortion.

Some comparatively fundamental discoveries to be discussed presently have enabled me to provide the present apparatus, which is much smaller, less sensitive to externally induced vibration, and substantially insensitive to attitude. In addition, the apparatus of this invention is considerably less expensive and involves fewer parts, and in some respects at least gives a superior result.

In general, the apparatus is comprised of an electromechanical transducer ltl which serves as a driver, and a similar unit 12 which acts as the pickup. These two units are interconnected by a pair of springs 14 and 16 which are in stretched condition. The driver It) and pickup 12 are mounted in spaced relation upon a channel strip 18 of sheet aluminum for instance, which in turn is supported within a boxlike housing 20 in such manner that the inner channel 18 is free to float. The inner channel is therefore comparatively insensitive to externally induced vibration, this being accomplished by suspending the inner channel 18 from the outer housing lit by means of four short coil springs 22 which connect at their outer ends to the housing 2%, their inner ends being connected to the channel strip 18.

The housing 20 is provided at its longitudinal edges with outwardly turned mounting flanges 24 which may have holes punched therethrough at any appropriate location. Input and output terminals 26 and 23 respectively are also carried by the housing 2% so that they are available from the outside, the input terminals at 26 being connected by means of flexible insulated wires 3% to the driver unit 19, similar flexible wires 32 serving to connect the pickup unit 12 to the terminals at 28.

The pickup unit, which is similar to the driver, is best illustrated in FIGS. 3 and 4. it is made up of a sheet metal angle bracket 34 having a base portion 36 fastened to the channel strip 18 by means of a screw 38 and an outstanding portion 4% at right angles thereto provided with a pair of side-by-side rearwardly extending tubular housings 42 and 44 having conical end caps 46. Each of these tubular members has an internal shoulder 48 and a headed sleeve 5%} which is pressed into the mouth of the housing member 42 so as to clamp a dead rubber vibration absorbing disc 52 between the shoulder 48 and the inner end of the sleeve 59, the operation of pressing the sleeve into place serving both to clamp the rubber disc 52 and also to secure the housing member 42 in assembled relationship to the member th of the mounting flange 34.

A highly compliant Wire 54, formed of beryllium copper for instance, has one end extended into a small diameter tube 55 which is then swaged and bent to form a small hook 56 integral with one end of the support wire 54. A small diameter annular permanent magnet 58, formed preferably of one of the ceramic magnetic materials, is slid over and adhesively secured to the rearward portion of the tube 55.

The magnet 58 is positioned slightly in front of the open mouth of the sleeve 50, and the wire 54 extends through a tight fitting perforation in the rubber disc 52, through the housing 42 and through a perforation at the center of the end cap 46 where it is bent over and soldered in place externally.

The assembly within the adjacent tubular housing 44 is substantially identical to that just described, the hook,

of this unit being indicated by the distinguishing numeral6tl.

A stack of laminations 62 is spaced slightly in front of the bracket portion 40 by a layer of plastic material 64, this assembly being secured to the bracket by means of a clip 66 and rivet 68.

As is best seen in FIG. 4 the laminations are punched so as to provide a central leg 70 attached at one end to a rectangular surrounding structure having legs 72 and 74 parallel to the central leg 75?. A coil of wire 76 is slid over the central leg 70 so that this central leg forms one pole of a magnetic circuit, whereas one of the outside parallel legsin the present instance, 74-forms the opposite side of the circuit. These parallel legs 74, and 70, provide a magnetic gap, and both of the annular permanent magnets 58 and 60 are positioned within this gap, one on each side of the mounting clip 66. The other outside leg 72 and the remaining portion of the surrounding magnetic assembly form a symmetrical structure and act as a shield to inhibit stray external magnetic fields from influencing the current generated in the coil 76 by movement of the magnets 58 and 69.

The driver unit at '19 may be considered as essentially identical to the pickup unit just described, excepting that since shielding against stray magnetic fields is of less consequence, a simple U-shaped laminated structure 80 may be used in place of the enclosing circuit provided in the pickup unit. In the driver an open coil 82 is slid over one leg of the U-shaped laminated structure, and the small annular permanent magnets (equivalent to the magnets 58 and 60) are disposed between the legs of the U, the laminations being secured to their mounting bracket 84 by means of a clip 86 and rivet 88, the mounting bracket in turn being attached to the channel 18 by a screw 96*.

The four annular magnets are polarized transversely so that the north and south poles are equidistant from the adjacent poles formed by the laminations just described. Preferably this is accomplished by assemblying the hooks S6 and magnets 53, or the equivalent, to the support Wires, and then passing the support wires through the rubber dampers 52 and out through the perforations in the end caps 46 and securing the support wires in place by soldering to the caps 46. Subsequently this partially assembled driver or pickup is placed into a magnetizing fixture where the magnets are magnetized and properly polarized in place. Thereafter the laminated structure having the electric coil thereon is attached to complete the driver or pickup subassembly. By magnetizing the magnets in place, the difiiculty of properly orienting the magnets during assembly is avoided.

The springs 14- and 16 have hooks 92. which engage the hooks 56 and 6t), and the equivalent hooks 61 and 63 at the driver when the springs are stretched. The spring hooks and the driver and pickup hooks should interengage closely so as to prevent there being lost motion at the connections. Furthermore, it is best if the hooks 92 of the springs are so formed that the engagement between these hooks and the hooks on the driver and pickup units are at the axis of the springs.

Each of the springs 14 and 16 is made up of two spring elements which are secured together in end-to-end axial alignment. The two spring elements for each spring are substantially identical but differ in that one is wound in a clockwise direction, whereas the other is wound counterclockwise.

At their adjacent ends, the spring elements are provided with hooks 94 which are joined by a connector 96. This connector is made from a short length cut from a piece of aluminum tubing through which the hooks are extended. With the hooks in place, the piece of aluminum tubing is flattened and squeezed sufiiciently to embed the hooks in the aluminum and provide a solid light weight connection with no lost motion. The hooks 94 and connector 96 should be at the axis of the spring elements.

The springs 14 and 16 differ in that one of these has a somewhat longer transmission time for vibrations. This may be accomplished as shown, by providing springs which are substantially identical excepting that one (16) has more turns than the other (14) If desired, the springs may have the same overall physical length, in which case the time of transmission of one may be made longer by forming it so that it has coils of a slightly larger diameter. Everything else being equal, the larger the diameter of the coils of the spring the slower the rate of transmission of vibrations from one coil to the next. Increasing the coil diameter, however, has the disadvantage of increasing the impedance of the spring and this in turn lowers the high frequency response. A similar alteration of transmission characteristics may be caused by varying the wire diameter. As a practical matter, it appears to be easier to use springs which are otherwise identical excepting for their overall length.

Reducing the helix angle of the springs is advantageous from the standpoint of vibration transmission, and also has the very important practical advantage of permitting the maximum number of turns, and hence maximum time delay, for a given overall length of spring. In order to obtain a low helix angle, or in other Words a close interturn spacing, I have found it to be a great advantage to wind the springs with a backward component so that the turns of the springs press tightly against each other. Subsequently, when the springs are stretched between the driver and pickup units, they are under sufficient tension to insure their remaining substantially straight, and in addition the interturn spacing is at a practical minimum. It should be appreciated, however, that the spacing between the turns must be positively maintained. If adjacent turns should touch each other at any point, the turns in contact will vibrate as a single turn having twice the rotational inertia of spaced turns. The result of this is to sharply reduce the high frequency transmission response of the spring and to generate distortion.

This apparatus operates as follows. Any sound system, such as a record player, electric organ, radio, or the like supplies its output signal both to a speaker and to the terminals 26 of the driver unit It). This energizes the coil 62 and causes the magnets connected to the hooks 61 and 63 to vibrate rotationally. The vibration is thus communicated to the springs 14 and 16 and travels down the springs so that it arrives at the opposite ends after an appreciable time delay, which is somewhat greater for the longer spring 16. When the vibration reaches the hooks 56 and 60 and the magnets 58, the major portion of the vibration is reflected back through the springs, while a minor portion thereof causes rotational vibration of the magnets 58, thus generating a signal in the coil 76 which is supplied to the terminals 28.

In the manner explained more fully in the previously referred to patent application, the output terminals 28 are connected to the input of a preamplifier which supplies the reverberated signal either to the same amplifier speaker system previously mentioned, or to a separate amplifier speaker system. In either event a listener hears both the direct signal and the reverberated signal.

The terminating impedance, determined principally by the rotational inertia of the magnet 58 and hook 56 and the compliance of the wire 54 is matched to the impedance of the first turn of the spring on such a basis that, as previously stated, the major portion of the spring vibration is reflected while a minor portion vibrates the magnet at the pickup and its wire support 54 and is largely absorbed by the rubber disc 52. The reflected portion of the vibration returns to the driver unit where it is partially absorbed and partially reflected back to the pickup and so on, so that with each successive appearance of the vibrations at the pickup the output signal becomes more attenuated until it finally becomes inaudible.

I have found that a satisfactory time for transmission of the signal from one end of the springs to the other is about 37 milliseconds for the long spring 16 and about 29 milliseconds for the short spring 1 A satisfactory rcverberation time is obtained with an attenuation of about 60 db in two seconds for the long spring and in 1.75 secends for the short spring. To some extent these characteristics are a matter of choice, but it has been observed from extensive listening tests that the delay time can be reduced below what is generally believed to be optimum if two or more springs are used simultaneously. Apparently the ear is not sensitive to the decay times when two or more channels of irrationally related delay times have the r outputs of equal or nearly equal amplitudes superimposed, and this has led observers to the false conclusion that longer than necessary delay times are advisable, probably because there was not sufficient understanding of the interrelationship of reverberation time and delay time. If a single channel is used, the listener usually prefers that its delay time lie within the range selected for the three springs in the apparatus of the previously mentioned application.

In general, as compared with the apparatus of the previously mentioned patent application, the transmission times have been considerably reduced. This, together with 'a careful proportioning of the delay times of the two springs, results in highly satisfactory results even though the springs have a less delay time and even though only two springs are used.

By using tension springs in which the turns have a mutually interturn compressive component and then stretching these springs to separate the turns slightly, -I have found that reliable separation of the turns can be achieved at a quite low helix angle. In other words, the turns can be somewhat closer together while still insuring a positive turn separation than is possible as a practical matter when the springs are wound with the turns separated in the free condition.

Although the springs have different free lengths, their stretched length is the same. This makes it possible for the driver arm-atures to be Within the same magnetic gap structure, thus resulting in a simple low cost assembly. The same is, of course, true of the pickup structure.

The ultimate result of this is a configuration in which the springs are in straight stretched condition. This greatly reduces the overall size of the apparatus and the space required for its installation. Furthermore, it will be appreciated that the straight springs under tension do not swing from side to side because of movement of the whole unit as do dependent springs. Also, the straight stretched spring arrangement here described is not sen sitive to attitude, since the springs under tension will always extend in essentially straight lines between the driver and pickup units regardless of the attitude of the unit.

The lower manufacturing cost, greater freedom in mounting location and attitude, lesser space requirements, and reduced sensitivity to vibration all contribute to the provision of a unit having a much greater practical utility.

Having described my invention in connection with a specific practical embodiment thereof, it will be appreciated that changes may be made in the structure without departing from the scope or spirit of the invention, and that, therefore, the scope of this invention is to be measured by the scope of the following claim.

aving described my invention, what I claim as new and useful and desire to secure by Letters Patent is:

An apparatus for providing reverberation of an electrical signal comprising electro-mechanical driver means, said driver means having electrical circuit means with input electrical terminals therefor and a plurality of movable elements, said electrical circuit means operating to vibrate said movable elements when an audio frequency signal is applied to said input terminals; mechanicoelectric pickup means, said pickup means having a plurality of rnovable elements and electrical circuit means having output electrical terminals therefor, said pickup means operating to generate an electrical signal and apply the last said signal to said output terminals when its movable elements are vibrated, a plurality of coil springs individually mechanically connected at one end to the movable elements of said driver means and at the other end to the movable elements of said pickup means and supported solely therebetween in parallel positions by their end connections to said movable elements, said springs having appreciably different effective lengths with respect to their rate of vibration transmission, said springs being of the type in which adjacent turns exert an appreciable uniform compressive force against each other when said springs are free, said springs when free being physically substantially shorter than the distance between said driver and pickup movable elements, and said springs being stretched when connected to said movable elements so as to be substantially straight and so as to effect a reliable interturn separation between all of the turns of said springs.

References Cited in the file of this patent UNITED STATES PATENTS 2,437,445 Stack Mar. 9, 1948 2,493,638 Olson Ian. 3, 1950 2,967,447 Hanert Jan. 10, 1961 2,982,819 Meinema May 2, 1961 

